1. Field of the Invention
The present invention relates to a stepping motor drive unit for optimizing a torque relative to a load.
2. Description of the Related Art
A conventional drive circuit for a two-phase stepping motor is shown in FIG. 1. A stepping motor comprises energizing coils 1 and 2 each responsible for one phase, transistors 3 to 10, feedback resistors 11 and 12, amplifiers 13 to 16, a power supply 17, a reverse-flow prevention diode 18, a pulse control circuit 19 for controlling forward and reverse rotations of a motor, a CPU 20, a backup capacitor 21, and a reference voltage source 22.
The operation of the stepping motor drive circuit having the foregoing circuit elements will be described briefly. The CPU 20 informs the pulse control circuit 19 of the number of drive steps. The pulse control circuit 19 turns on or off the transistors 3 to 10 so that certain pulsating current flow into the coil 1 or 2 in a given direction. That is to say, a constant current drive circuit composed of the transistors 3 and 8, feedback resistor 11, amplifier 15, and reference voltage source 22 provides a pulsating current flow through energizing coil 1 in one direction. A constant current drive circuit composed of transistors 4 and 7, feedback resistor 11, amplifier 16, and reference voltage source 22 provides a pulsating current flow in the opposite direction through energizing coil 1. The same applies to energizing coil 2. A constant current drive circuit composed of transistors 5 and 10, feedback resistor 12, amplifier 13, and reference voltage source 22 provides a pulsating current flow through energizing coil 2 in one direction. A constant current drive circuit composed of transistors 6 and 9, feedback resistor 12, amplifier 14, and reference voltage 22 provides a pulsating current flow in the opposite direction through energizing coil 2. The constant current value is determined in accordance with the voltage value at the reference voltage source 22.
FIG. 2 is a side view showing a structure in which a stepping motor STPM is used as an actuator for driving a lens array in a camera. The actuator comprises a lens array receptacle 23, a ground plate 24, a lead screw 25, an offset spring 26, yokes 27 and 28 each having a coil responsible for one phase, and a rotor 29.
A stepping motor STPM composed of yokes 27 and 28 and rotor 29 is fixed to a ground plate 24. The lens array receptacle 23 is threaded so that the distance thereof from the ground plate 24 can be varied with rotation of the lead screw 25. The rotor 29 and lead screw 25 are mutually coaxial and fixed. The lens array receptacle 23 therefore moves along the lead screw with rotation of the stepping motor STPM. Assuming that the lens array receptacle 23 moves from an initial position 63 in a non-pressing direction 30 of offset spring 26, that is, in a direction in which the offset spring 26 does not press the lens array receptacle (the stepping motor rotates counterclockwise) or in a pressing direction 31 thereof, that is, a direction in which the offset spring 26 presses the lens array receptacle (the stepping motor rotates clockwise), a quantity of movement, a load torque applied to the shaft of the stepping motor STPM, and a torque of the stepping motor STPM have the relationships shown in the graph of FIG. 3. In FIG. 3, the abscissa indicates a quantity of movement 32, and the ordinate indicates a torque 33. It is seen from FIG. 3 that even when the load torque 34 applied to the motor shaft becomes maximum, the torque 35 of the stepping motor STPM remains unaffected.
As long as the number of revolutions and input current of the stepping motor STPM are constant, the torque of the stepping motor STPM remains constant. The load torque 34 applied to the motor shaft varies linearly according to a pressing force provided by the offset spring 26. In other words, the load torque 34 increases when the stepping motor rotates clockwise, and decreases when the stepping motor rotates counterclockwise.
However, when the load torque applied to the motor shaft varies, the response curve plotting a trajectory along which the stepping motor STPM makes the first step becomes as shown in FIG. 4. When a first step start signal 37 is generated, if the load torque applied to the motor shaft is large, then the response curve is plotted as indicated with 39. If the load torque applied to the motor shaft is small, then the response curve is plotted as indicated with 38 to show a large overshoot. The next step start signal must therefore be 10 generated according to the timing at which the response curve stabilizes. This poses a problem in high-speed operation of the stepping motor STPM. Supposing a second step start signal would be generated according to the timing indicated with the response curve 38 in FIG. 4, the stepping motor STPM rotates in relative movements and becomes unstable. At worst, a loss of synchronism occurs.